In the drilling of bore holes, such as well bores, mine bores or other bore holes into or through earth formation, utilizing a rotary bit, the drilling or penetration rate efficiency depends upon the weight or axial force applied by the drill bit and the speed of rotation. Various types of formations and types of bits as well as size or diameter of the bits require different weight or load on the bit and different speeds for maximum penetration rate.
Drilling of bore holes using drilling fluid, either mud or gas, also requires that the drilling fluid be circulated through the drill string and bore hole to flush cuttings and cool the bit.
When driving a rotary bit utilizing an in-hole motor, either of the positive displacement or the turbine type, which is driven by the drilling fluid, the motor speed is a function of the circulation rate and cannot always be varied to adjust the speed of rotation of the bit due to the requirements for adequate drilling fluid flow, and in the case of low-speed motors due to high friction if the circulation rate is increased to increase drill speed.
Heretofore, therefore, speed changers have been proposed for use in drilling with various in-hole motor drills. The proposal is relatively simple in the case of turbines, which operate, notoriously, at relatively high speed, as compared with in-hole motors of the positive displacement type, such as, for example, the helicoidal motors. The simplicity, in the case of turbine type motor drills to reduce or increase speed of the drill at any given motor speed is attributable to the axial alignment of the motor or turbine shaft and the drive shaft for the drill or bit. However, in the case of in-hole motors of the helicoidal type, the drive end of the helical rotor partakes of a compound motion, the center of the rotor rotating in one direction while orbiting in the opposite direction.
There is a need, however, in the use of in-hole motors of the helicoidal type, for enabling rotation of the drive shaft at a speed either greater or less than the speed of the rotor. Relatively small diameter drills are rotated at higher speeds than larger diameter drills to obtain optimism peripheral speed in a given formation.